Common beach crab

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Common beach crab
Common beach crab (Carcinus maenas)

Common beach crab ( Carcinus maenas )

Systematics
Subordination : Pleocyemata
Partial order : Crab (Brachyura)
Superfamily : Portunoidea
Family : Carcinidae
Genre : Carcinus
Type : Common beach crab
Scientific name
Carcinus maenas
( Linnaeus , 1758)

The common beach crab ( Carcinus maenas ), also simply called the beach crab , is a very common species of crab on temperate and subtropical coasts . Its original area of ​​distribution is the Atlantic coast of Europe and North Africa , but it was introduced into other regions through human activity, so that it is now considered to be almost worldwide. It is an adaptable omnivore , lives in salt - and brackish water and can be used as so-called invasive way in places outside their ancestral homeland a great influence on the composition of the domestic animal - and plant life have.

features

Preparation of a sea crab (top view)

Beach crabs have the typical external characteristics of a crab. The exoskeleton that surrounds the entire body forms a hard shell and makes up almost 40 percent of the animals' body mass. The one-piece back shield ( carapace ) has a diameter of up to 86 mm in males and up to 70 mm in females . However, only a few individuals reach this size, since body growth is usually stopped before a carapace diameter of 60 mm with a final moult . The spine shield is sawn on the front edges with 5 teeth on each side. In the area of ​​the stalked compound eyes , the foremost tooth forms a pit in which the respective eye can be folded in to protect it. The frontal edge has three rather blunted teeth.

Beach crab from the front. The right 3rd maxilliped (Mxp3) is moved to the side so that part of the mouth field with the right mandible (Mdbl) can be seen. The right stalk eye (Oc) is folded into the eye pit, the 1st antennae (Ant1) are folded into the protective gaps. The left 2nd antenna (Ant2) and the left scissor bone (P1) are also clearly visible. The right scissor bone is completely missing.
Young beach crab

coloring

The color depends mainly on the age and the time that has passed since the last molt. In animals that still change their cuticula regularly, the upper side of the body is usually colored in dark green tones and turns into a dull yellow on the lighter underside of the body. The longer a crab can do without a molt, the sooner the underside turns deep red with a correspondingly darker, brownish upper side, presumably a consequence of the persistent denaturation of the pigments in the integument with increasing age . This is especially the case with older animals that shed their shell less often or have already done the final moult. It is assumed that in many larger individuals the change of the cuticle is also delayed so that thicker armor can be developed, which should be of particular importance in the competitive fights of the males. At the same time, however, in comparison to the greenish colored conspecifics, signs of wear and tear and colonization by barnacles increase as well as the tolerance to changed environmental conditions (such as fluctuations in the salt content in the surrounding water). Young crabs with a carapace of only a few millimeters differ significantly in their body markings from older animals and can adapt their color to their surroundings to a limited extent. In addition to the green-brownish basic color, they often have conspicuous white, black and red patterns, so that they are more difficult to see between pebbles and fragments of mussel shells . In addition, the young stadiums always wear a pattern of dark and light bands on their legs for a certain period of time. However, this color variability disappears with increasing age.

extremities

Pleon of a female beach crab
Pleon of a male beach crab

The pair of scissors on the scissor legs (converted first pair of legs ) is strong. As with many crabs, a pair of scissors is usually more or less developed and is often referred to as "cracking scissors" in German-speaking countries. Most beach crabs wear these on the right side of their bodies. However, animals can also be found in which no snap scissors are formed, i.e. both scissors are of the same type. The last pair of legs (peraeopods) has the shape common to crabs, which distinguishes them from similar crab species such as the common swimming crab . In these, the rearmost leg is designed differently and the last leg is widened like a paddle.

The third maxillipedas (foremost pair of mouth tools) completely cover the mouth area when at rest, so that the remaining five pairs of mouth tools can only be seen from the outside when the third maxillipedes are moved, e.g. B. during the eating process. As with most crabs, the two pairs of antennas used to perceive smells are relatively short. The first antennas are folded diagonally into the specially developed gaps.

The abdomen ( pleon ) of the female is rounded and wide, the male is more triangular and narrow. The segments  3–5 are fused together in the latter so that the segment boundaries can no longer be seen. The observer can distinguish the common beach crab from the closely related species Carcinus aestuarii mainly by the shape of the pleopods that are transformed into mating organs in the male : in the common beach crab these are curved, while in Carcinus aestuarii they tend to run straight. (See also the corresponding generic article .)

distribution

Distribution of the common beach crab. Blue: domestic distribution. Red: Introduced with invasive reproduction. Green: Introduced with potential spread. Black spots: more sightings (introduced).

The ancestral range of the common beach crab stretches from the coast of northern Norway to the Atlantic coast of North Africa and the coast of Iceland . For large parts it is one of the most common crab species here. In the Mediterranean , however, it is largely replaced by the closely related species Carcinus aestuarii .

The beach crab as a neozoon

It is assumed that different distribution mechanisms were important for the worldwide occurrence of the species. The first beach crabs probably reached new shores in the holey wooden hulls of merchant ships that were sailing between Europe and North America . Later, when wooden ships became less important for shipping, other mechanisms played a role. For example, it was possible to spread between the ballast loads (for example ballast water ) from ships.

The beach crab was first reported as a neozoon in 1817 near Massachusetts , from where it spread over almost the entire east coast of North America. It was discovered on the west coast of the United States in 1989 and has since spread over 750 kilometers along the coast in 10 years . In 2003 she reached the Patagonian coast of South America .

In Australia it was first discovered in Port Phillip at the end of the 19th century . Since then, the species has expanded its range along the Australian south coast including Tasmania .

In 1983 specimens of the species were found for the first time on the coast of South Africa near Cape Town , where they also multiplied and spread like an invasion.

After Japan were hybrids of Commons beach crab and Carcinus aestuarii that the forms from the region of the Strait of Gibraltar correspond introduced. These were also able to firmly establish themselves here.

There have been other sightings outside of their original home in many other places, but without the crab being able to develop populations of any significant size. However, based on the climatic and ecological demands of the species, it is likely that it will migrate to further areas. It is believed, for example, that the beach crab spreads to Alaska in western North America . The coasts of New Zealand are the only major potential range that has so far been spared immigration. The government there has taken extensive measures to prevent this in the future. For example, an information sheet was published for the population in order to identify the species quickly and to inform the responsible authorities in the event of a find.

Way of life

habitat

This stretch of coast in the Wadden Sea shows the typical habitat of the beach crab.

In areas with pronounced tides , beach crabs can be found in the upper tidal range, which falls dry for many hours at low tide , to 60 m deep waters, but prefer to be in shallow water and near the shore for most of the year. They colonize all semi-protected coastal types with not too strong surf and do not have any special requirements for the soil quality: beach crabs occur on sandy soil as well as on rock or silt . They often appear en masse.

Activity rhythm and adaptation to the coastal habitat

The rhythm of activity is mainly determined by the tides and the time of day, with the highest activity at high tide and at night. However, even during periods of low activity, some individuals go in search of food. When the tide is out, beach crabs either migrate with the returning water or look for hiding places on the beach. For example, they hide under stones or tufts of seaweed or dig themselves into the ground. They survive several hours of drying out unscathed, as the gill cavities can hold moisture well. Instead of a stream of water, the crustaceans can then use the scaphognathites (structures of the mouthparts) to generate a stream of air and thus supply the gills with new oxygen . This breathing activity can be recognized by the formation of water bubbles in front of the crab's mouth, together with a clearly audible bubbling noise. The younger specimens in particular, with a carapace of up to 3 cm wide, stay on the beach permanently, where they are only covered by water at high tide. If an animal remains in a small tide pool at low tide , it is exposed to a rapidly falling oxygen content in the water. Beach crabs counter this fact with a special behavior in which they hold the outflow opening of the respiratory water located in the mouth above the water surface. From time to time the flow of breath is reversed so that air is transported into the gill cavity in order to enrich the water it contains with new oxygen. In rare cases, beach crabs also dig tunnels in salt marshes that are sometimes not flooded for days. Since the air in it remains cool and humid, they survive these conditions.

The species is euryhaline , i.e. That is, it can tolerate a wide range of different salt contents in the water without affecting body functions. A decrease in the salt content can, for. B. compensate for this by actively absorbing salts from the water through the gills. This also enables her to stay in the brackish water of estuaries . In order to avoid the cold temperatures in the winter months, beach crabs retreat from the nearby shore at this time and seek deeper waters.

Diet and predators

A beach crab moves ashore

The cancer is not picky in his diet: He eats virtually anything that can be overwhelmed with the scissors, for example, various mollusks , polychaetes , cnidarians , echinoderms , fish and other crustaceans. Hard-shelled prey such as mussels and snails ( large periwinkle and others) are broken open beforehand with the cracking shears. Beach crabs also eat carrion and vegetable foods such as seaweed . The species is very voracious and can have a strong influence on the population density of the animal species that make up its prey range.

The animals themselves also have a large number of predators. For example, they are eaten by seabirds , fish and cephalopods and often represent an important part of the main prey for them. The crabs use their claws for defense. The defensive position typical of crabs is often taken, in which the pincers are spread wide apart and beaten together when the enemy approaches. The shell also offers the crabs a certain protection. A common escape reaction , however, is to run away, looking for a suitable shelter as quickly as possible. With its walking legs, it can move surprisingly quickly in the water as well as on land, and it shows the typical lateral movement for crabs: "dwarslöper". In a specimen with a 45 mm carapace width, a walking speed of up to 1 m / s was measured under water. In addition, the animals can change direction very quickly.

Growth bladders in which the first (right arrow) and second (left arrow) leg grow back. The triangles indicate the places where the respective extremity was thrown off.

Beach crabs, like all crabs, are capable of so-called autotomy , i.e. That is, they can throw off extremities (for example the scissor bones) if this enables them to escape from predators or if the extremity in question is damaged. This is often the case with the beach crab, so that one can regularly find individuals with one or more extremities missing. This can then grow back over the course of several moults. If the beach crab loses its scissor leg with the snap scissors as a result of an autotomy, the scissors of the remaining leg transform into new snap scissors in the course of the moulting.

Social behavior

Beach crabs fight in the silt

Beach crabs often show aggressive behavior towards one another: If two individuals surprisingly meet, they often threaten one another. It is not uncommon for males to fight, especially to gain access to the females. However, the animals can hardly inflict serious injuries due to their armor. Exceptions are encounters between animals with significant differences in size and age. Beach crabs often eat each other here. Crustaceans in particular, in which the carapace has not yet hardened after molting (" butter crabs ") are susceptible to this and can be overwhelmed even by smaller conspecifics. In areas where many young animals settle, this cannibalistic behavior even seems to regulate population densities. But just as often one can find beach crabs that literally “sit on top of one another” in their hiding places in the intertidal zone and behave completely peacefully. With such an accumulation of individuals, moulting is suppressed, probably to avoid cannibalism.

Reproduction and development

Female beach crab with eggs

There are regional differences in the mating seasons given the large range of the sea crab. Egg-bearing females can be found in some areas all year round. Under less favorable conditions, there are defined mating seasons in which the females gather at certain places to be mated . Here they wait for the males, between whom there are usually fights. Since mating can only take place when the female molts, the males carry a female that has been secured with them until then and may have to defend her again from competitors.

The up to 185,000 eggs are carried around by the female in a ball under the pleon. When threatened, they keep their extremities close to the body so that the legs are placed protectively over the ball of the egg. The larvae hatch after up to four months, depending on the ambient temperature, and are then released into the free water. For this purpose, most females withdraw from the waters near the coast or leave the brackish water of estuaries in order to release the larvae into the salty sea water. The hatched from the egg, less than a millimeter wide Prezoea larva is transformed within minutes Zoea larva, which then becomes free-floating plankton belongs. After molting into the Megalopa larva, it becomes a young crab that is barely more than a millimeter wide and lives on the ground from now on.

How fast the crabs grow depends not only on the food supply, but also above all on temperature. The sexual maturity is mm in the male at a Carapaxbreite 25-30, mm is reached in the female 15 to 31 The age is between 5 and 10 years. In regions with low ambient temperatures, sea crabs grow slower, reach sexual maturity later and have a longer life expectancy.

Parasites

The species belongs to the hosts of the parasite Sacculina carcini , which sterilizes the infested crab, prevents further growth and weakens it so much that sooner or later the infestation leads to the death of the host. In order to control the beach crab in introduced regions, consideration has been given to releasing the parasite on affected coasts of North America. Studies have shown, however, that this is not an optimal means of combating it, as native crabs can also be attacked. Other known parasites are flatworms , cordworms and woodlice , but little research has been done on them.

Common beach crab and human

The common beach crab is a popular animal observation object for many holidaymakers on European coasts. If they are startled, they assume their threatening position and rear up. If you try to catch them with your bare hands, they often pinch with their scissors, which can cause painful but usually harmless bruises or cuts to the hand. Females are less aggressive and tend to compact in such cases, similar to the protective reflex they show while eggs are carried under the body.

The common species is one of the Wadden Sea's Small Five .

Cultural meaning

Although it is one of the most famous crustaceans, it is rarely represented culturally by humans . One example are Icelandic coins still in circulation that show a beach crab on the reverse.

Economical meaning

The species is meaningless to commercial fisheries worldwide. It is only caught and eaten locally. For example, there are North American restaurants that have sea crab on their menu. In 19th century Great Britain , they were mainly eaten by the poor population along the coast, but there are also reports of large quantities ending up in the London markets. Today, however, they are used more as fishing bait there . In many places, beach crabs are an unpopular bycatch when fishing and the mussel and crustacean fishery and are considered pests because they feed themselves on the catch. On the east and west coast of North America, in particular, they reduce the catch quotas of the resident shrimp fishermen by displacing a native species of shrimp.

The beach crab in research

Since the species is widespread and common, has an easily manageable body size and is a typical representative of the crab in terms of both lifestyle and morphology , the common sea crab has long been established as the standard animal model in crab research. The research areas range from ecological and developmental studies to special physiological , neurobiological and biochemical experiments .

Etymology and systematics

Ancient Greek vase depicting the attack of the crab Karkinos on Heracles

The species was first described by Carl von Linné in 1758 . He initially gave it the scientific name Cancer maenas . The British zoologist and marine biologist William Elford Leach gave the final genus name Carcinus in 1814. It is a Latinization of the Greek word “Karkinos” (καρκίνος), which simply means cancer or crab in German. Karkinos is also the name of a crab from Greek mythology who was a companion of the Hydra in the fight against Heracles . The epithet maenas is derived from the maenads ( lat. Sg. Maenas, -adis), an ancient cult of women who worshiped the Greek god Dionysus .

Although it is one of the best-researched marine animals , little is known about the family position of the common sea crab. This is because the phylogeny of the crabs is currently changing a lot and has only recently been investigated in more detail. Current family trees assign them to a family named after their genus ( Carcinidae ).

literature

Book literature

  • P. Brohmer (Greetings), M. Schaefer: Brohmer, Fauna von Deutschland: an identification book of our domestic animal world. 21st edition. Source and Meyer Verlag, Wiebelsheim 2002, ISBN 3-494-01326-8 .
  • A. Kaestner (Gre.), H.-E. Gruner: Textbook of special zoology. Volume 1: Invertebrates. Part 4: Arthropoda (without Insecta) . Gustav Fischer Verlag, Jena / Stuttgart / New York 1993, ISBN 3-334-60404-7 .
  • G. Quedens: Beach and Wadden Sea: Animals and plants on the North and Baltic Seas; a habitat guide. 7th edition. BLV Verlagsgesellschaft, Munich 1998, ISBN 3-405-15108-2 .
  • V. Storch, U. Welsch: Systematic Zoology. 6th edition. Spectrum Academic Publishing House, 2003, ISBN 3-8274-1112-2 .
  • V. Storch, U. Welsch: Kükenthal - zoological internship. 26th edition. Spectrum Academic Publishing House, 2009, ISBN 978-3-8274-1998-9 .

Scientific publications

General biology

Neozoon problem

Systematics

Web links

Commons : Common Beach Crab ( Carcinus maenas )  - Collection of images, videos, and audio files

Individual evidence

  1. ↑ Bryan 1961.
  2. ↑ a b c d e f g h Crothers 1967.
  3. ↑ Bjarne 2004.
  4. ↑ Reida 1997th
  5. ↑ a b c d e f g h i Crothers 1968.
  6. ↑ Hogarth 1978.
  7. ↑ a b Storch 2009.
  8. ↑ Kaestner
  9. ↑ a b Abby-Kalio 1989.
  10. ↑ Pynn 1998th
  11. ↑ Brohmer 2002.
  12. ↑ Kaestner
  13. ↑ a b c Yamada 2001.
  14. ↑ a b Carlton 2003.
  15. ↑ Classes 2007.
  16. ↑ a b Management plan for European Green Crab. (PDF; 529 kB) November 13, 2002, accessed January 20, 2013 .
  17. ^ Hidalgo 2005.
  18. ↑ Thresher 2003.
  19. ↑ Grosholz 1996.
  20. ↑ Marine Pest Guide. (PDF; 206 kB) (No longer available online.) June 2006, archived from the original on February 18, 2013 ; Retrieved January 20, 2013 .
  21. ^ Non-indigenous aquatic species of concern for Alaska. Fact Sheet 1. (No longer available online.) April 2004, archived from the original on August 25, 2014 ; Retrieved January 20, 2013 .
  22. ↑ a b Quedens 1998, p. 62.
  23. ↑ Laurent Seuront: Microplastic leachates impair behavioral vigilance and predator avoidance in a temperate intertidal gastropod. In: Biology Letters. 14, 2018, p. 20180453, doi: 10.1098 / rsbl.2018.0453 .
  24. ↑ Grosholz 1995.
  25. ↑ Grosholz 2005.
  26. ↑ a b Ropes 1968.
  27. ↑ Kaestner 1993, p. 967.
  28. ↑ Moksnes 1998th
  29. ↑ a b c MarLIN. (No longer available online.) Archived from the original on August 25, 2014 ; Retrieved November 1, 2012 .
  30. ↑ van der Meeren 1994.
  31. ↑ Berril 1982.
  32. ^ Goddard 2005.
  33. ↑ FAO Nominal Catches: Carcinus maenas . Retrieved December 31, 2010 .
  34. ↑ McDonald 2001.
  35. ↑ Ng 2008.
  36. ↑ Karasawa 2008.
  37. ↑ Schubart & Reuschel 2009.
  38. ↑ Spiridonov 2014.
This version was added to the list of articles worth reading on October 13, 2012 .